Smoke detectors of the ionization type are well recognized for their ability to detect fast developing fires, which have little smoke, but produce large quantities of small product of combustion particles. However such detectors are often unable to detect, in a reasonable time, fires of the slow smouldering type, which produce large quantities of smoke, but a lesser amount of small product of combustion particles than a fast developing fire. Therefore such detectors are less effective than optical detectors in detecting slow smouldering fires, and some manufacturers cannot meet the requirements of some regulatory bodies that establish standards of performance of smoke detectors.